Are Follicle-Stimulating Hormone (FSH) and Luteinizing Hormone (LH) suppressed during pregnancy?

FSH and LH Suppression During Pregnancy

Yes, FSH and LH are suppressed during pregnancy due to negative feedback mechanisms from elevated placental hormones, with FSH being more profoundly suppressed than LH.

Mechanism of Suppression

During pregnancy, several hormonal changes occur that affect the hypothalamic-pituitary-gonadal axis:

• Human Chorionic Gonadotropin (hCG):

  • Rises rapidly in early pregnancy
  • Structurally similar to LH but with longer half-life
  • Takes over the role of LH in maintaining the corpus luteum

• Estrogen and Progesterone:

  • Progressively increase throughout pregnancy
  • Exert strong negative feedback on the hypothalamus and pituitary
  • Inhibit GnRH pulsatility and gonadotropin release 1

Evidence of Suppression

Multiple studies have confirmed the suppression of both FSH and LH during pregnancy:

• Basal levels of both FSH and LH are low or undetectable during pregnancy 2
• FSH levels remain consistently low throughout pregnancy, with minimal fluctuation 3
• LH levels measured by specific assays (that don't cross-react with hCG) show significant suppression 4

Differential Suppression Patterns

The suppression pattern differs between FSH and LH:

• FSH suppression:

  • More profound and complete
  • FSH response to GnRH/LHRH stimulation is almost completely inhibited by the 3rd week of gestation 5
  • Remains below the lower normal limit from the 3rd week of pregnancy onward 5

• LH suppression:

  • Initially less complete than FSH
  • Progressively decreases during pregnancy
  • LH response to GnRH/LHRH is completely inhibited by the 5th week of gestation 5
  • Small, short-lived responses to LHRH may be observed in some pregnant women 2

Clinical Significance

Understanding this suppression is important in several clinical contexts:

• Differential diagnosis: Distinguishing between functional hypothalamic amenorrhea (FHA) and pregnancy
• Fertility treatments: When evaluating women with history of pregnancy loss or infertility
• Hormone therapy monitoring: For women requiring hormone therapy during pregnancy

Important Considerations

• Despite the suppression of pituitary FSH, bioactive FSH-like activity actually increases during pregnancy due to placental production 6
• This placental FSH differs from pituitary FSH and is not detected by standard immunoassays
• The suppression of gonadotropins appears to be primarily mediated by the continuously elevated levels of progesterone in the presence of appropriate estradiol levels 5

Common Pitfalls

• Misinterpreting hCG cross-reactivity: Some LH assays may cross-react with hCG, giving falsely elevated LH readings during pregnancy
• Overlooking the role of placental hormones: The placenta takes over hormone production from the pituitary-ovarian axis
• Confusing bioactive vs. immunoreactive FSH: While immunoreactive FSH (I-FSH) is suppressed, bioactive FSH-like activity (B-FSH) increases during pregnancy 6

This suppression of FSH and LH represents a normal physiological adaptation during pregnancy, allowing the placenta to take over the hormonal control of pregnancy maintenance.